Thermoplastic polyurethanes (TPUs) are of great technical significance because of their good elastomer properties and thermoplastic processability. Kunststoff Handbuch [G. Becker, D. Braun], volume 7, “Polyurethane”, Munich, Vienna, Carl Hanser Verlag, 1983, for example gives an overview of the production, properties and applications of TPUs.
TPUs are mostly composed of linear polyols (macrodiols), such as polyester diols, polyether diols or polycarbonate diols, organic diisocyanates and short-chain, mostly difunctional alcohols (chain extenders). They can be produced continuously or discontinuously. The best-known production processes are the belt process (e.g., British Patent Pub. No. GB1057018) and the extruder process (e.g., German Patent Pub. No. DE1964834), the entire contents of each of which is hereby incorporated herein by reference.
The thermoplastically processable polyurethane elastomers can be produced either stepwise (prepolymer metering process) or by the simultaneous reaction of all components in one stage (one-shot metering process).
In the use of aliphatic light-stable transparent materials, materials based on H12-MDI which, however, have the disadvantage of a poor setting behaviour, are mostly used. Aliphatic light-stable transparent TPUs based on other materials such as e.g. HDI (hexamethylene diisocyanate) are therefore desirable as diisocyanate, these TPUs however being too cloudy for such applications.
International Patent Pub. No. WO 02/050151, the entire contents of which are hereby incorporated herein by reference, describes transparent TPUs based on ethylene oxide polyols, or ethylene oxide-propylene oxide polyols with mixed chain extension, wherein one of the chain extension components is selected from the group ethylene glycol, diethylene glycol or 1,3-propanediol. In the examples, only TPUs based on aromatic isocyanates that are not light-stable are described.